Result of reducing the length of the nichrome wire of the heater
By shortening the length of the nichrome wire in the heater, the resistance of the wire is reduced. Because resistance is proportional to length. But, as the potential difference in the electric line remains constant and the resistance of the wire decreases, the current increases.
Since the heat generated is proportional to the square of the flow rate, decreasing the resistance will produce less heat, while increasing the flow rate will produce more heat. As a result, shortening the heater coil will generate more heat. If the same amount of current flows in the coil, the shorter the coil, the less heat will be produced.